Fluid flowmeter with heated resistance bridge circuit



p 9 1953 w. G. MIDDLETON ETAL 2,650,496

FLUID FLOWMETER WITH HEATED RESISTANCE BRIDGE CIRCUIT Filed May 17, 1948 2 Sheets-Sheet 1 METER GENERATOR \W;\\REGULATOR HLTER lies AMPLWWER A GENERATOR METER 1e 1 INVENTORS W.G.M|DDLETON1 ATTORNEYS p 1, 1953 w. G. MIDDLETON ET AL 2,650,496

FLUID FLOWMETER WITH HEATED RESISTANCE BRIDGE CIRCUIT" Filed May 17, 1948 2 Sheets-Sheet 2 ILTE R IMPEDA NCE AMPLIFIER REGULATOR GENERATOR METER |6 |4 i 1' 1/ I g /36 IOA- I :--3 --34 35"l I l l I INVENTORS w.c .MIDDLET'ON MRjOR E ATTORNEYS Patented Sept. .1, 1953 UNITED STATES PATENT OFFICE FLUID FLOWMETER WITH HEATED RESISTANCE BRIDGE CIRCUIT Application May 17, 1948, Serial No. 27,404

14 Claims.

This invention relates to apparatus for measuring the velocities, and/ or rate of flow of fluids. In one specific embodiment it employs a Wheatstone bridge circuit with two arms or branches protected or enclosed from and the other two arms exposed to flowing fluid. In another specifio embodiment only one arm is exposed to the flowing fluid and the other arm is exposed to a non-flowing portion of the same fluid. In some embodiments the velocity or rate of flow is obtained from a consideration of the deflection of a galvanometer, or the like, connected across the flow measuring elements and showing the power required to maintain a certain temperature dif ferential between the fluid and at least one of the flow measuring elements.

Instruments of this general type in the prior art all depend for their results upon a measurement of the current through the galvanometer and as the deflection of the galvanometer is proportional, in general, to the cube of the main current, the instrument necessitates a very accurate ammeter if its galvanometer indications are required to have a fair degree of accuracy. For low rates of fluid flow these prior art devices become so inaccurate as to be inoperative, impractical or useless.

Instruments of the prior art also present troublesome adjustments which have to be made for variations in the temperature of the medium whose velocity or rate of flow is measured, and this is a considerable drawback when the apparatus is to be employed in deep oil or gas wells, or in water input wells used in secondary recovery programs.

The simplified Wheatstone circuit of the prior art, in which the velocity or rate of flow indication depends upon the cooling effect of the flowing fluid upon the exposed hot wire element, is obviously readily adaptable to high velocity measurements but not tolow velocity measurements since heating of the fluid occurs causing convection currents, in say, fluid flowing at the velocity of a few feet per minute, thus causing false indications.

It is preferred to use alternating currents in this flow meter which permits impedance elements to be employed in the non-temperature sensitive arms of the bridge circuit. It is further preferred to use two alternating current frequencies in this invention in a manner which permits balance of the circuit, as well as to automatically maintain and regulate the temperature of the flow measuring element above that of the fluid by a constant amount.

one o j ct of our invention is to pr id proved means for determining accurately the velocity and/or rate of flow of fluids by electrical measurements.

Another object is to measure low rates of flow by said means.

Another object is to provide means to block a conduit to flow except flow through the flow measuring instrument.

Numerous other objects and advantages will be apparent to those skilled in the art upon reading the accompanying specification, drawings and claims.

The present invention measures the velocity (non-vectorially) by means of a Wheatstone bridge circuit, and if the measurement ismade in a conduit, or the fluid is forced to flow through a conduit in the instrument, and said conduit has a known area (A) the rate of flow (Q) is a function of the velocity (V) according to the obvious formula:

One feature of the preferred Wheastone bridge employed in this invention is that by using impedance elements in the non-temperature sensitive portions of the circuit, an alternating current bridge can be used. In the bridge circuit, both temperature sensitive elements are afiected correspondingly by any change in the temperature of the fluid, while one temperature sensitive element will respond predominately to the velocity or rate of flow. The galvanometer may be self-recording, and can be calibrated to show velocity, or rate of flow, or both on separate scales.

A specific feature of this invention is tokeep the temperature of the velocity of flow measuring element above that of the fluid by a constant amount. Another specific feature of the invention is the provision of a method and means for supplying, or removing current, from the velocity, or flow measuring element, automatically in proportion to the increase, or decrease, in cooling effect of the flowing fluid.

The invention preferably comprises an im provement over the standard Wheatstone bridge hot wire flow meter whereby potential developed across the bridge due to unbalance is amplified, and such amplified potental is applied through a regulator to change the supply of electric heating current to the flow measuring element of the bridge in sufficient amount to maintain constant the temperature differential between that of the flowing fluid and flow measuring element of the active arm of the bridge.

Another feature preferred as useful to carry out the objects mentioned above comprises the use of two sources of alternating current applied at two different frequencies to the measuring arm of the bridge circuit, in such a manner that the two' currents remain separated, one energizing the balanceable bridge to supply a current proportional to the unbalance thereof, which current is used to regulate a second current of a different frequency so as to balance the entire circuit; the other regulated to supply power to maintain a constant temperature differential between the flowing fluid and flow measuring element.

Another feature preferred as useful to carry out the objects is an inflatable packer sealing means which seals the flow meterfat a'predetermined point in a well forcing all fluid flowing past that point to flow through the flow meter.

When a portable power source is used, the recorder and meter, as one unit,'can be suspended H on a line in a drill hole. The power source and recording may be at the surface and'only essential bridge elements suspended on a line in the drill hole. The inflatable packer'ineach case will have to be inflated'from the surface with air, water, or other fluid.

It is obvious that the principles on which this invention depends for its operation as a velocity or flow indicator are (1) the dependence of the cooling of a hot resistance element on the velocity of flow of the fluid in which the measuring element is immersed, and (2) the change of electrical resistance of the element with change in temperature.

Figure 1 is a block circuit diagram of one illustrative embodiment in which one of the temperature sensitive elements is shielded from the flowing fluid, but exposed to the non-flowing fluid;

Figure 2 is a block circuit diagram of a second illustrative embodiment in which both temperature sensitive elements are exposed to the flowing fluid, but havedifferent resistances;

Figure 3 is a block circuit diagram of a third illustrative embodiment in which temperature sensitive elements are exposed to the flowing fluid and both having approximately the same resistance;

Figure 4 is an elevational view of a flow meter embodying the flow measuring elements of any one of Figures 1 to 37in collapsed position;

Figure 5 is an elevational View of the apparatus shown in Figure 4 in inflated position at a predetermined point in a well, the well being shown in cross section.

In Figure 1, numerals band 1 represent temperature sensitive legs of the bridge circuit and are preferably electrical resistance heating wires of platinum while the other two legs comprise impedance elements 8 and 9, respectively. In this particular arrangement leg 7 is exposed to the fluid flow in a conduit I and is maintained at a temperature above that of the flowing fluid by a constant amount, while leg 6 is shielded from the flowing fluid, but exposed to non-flowing fluid in a blind branch 19A of the conduit IS, so that the fluid in H] and NA is the same fluid and at the same temperature, the only difference being that it flows through conduit H] but does not flow appreciably in branch iiiA. Element l measures the velocity or rate of flow, while element compensates for temperature changes of the fluid encountered as the flow meter is lowered into the drill holefluid. The temperature of resistance element 'lis maintained at a constant temperature differential above that of the fluid by means of electrical power supplied from generator l2.

The balanced circuit or system shown in Figure 1 comprises a bridge circuit, such as a Wheatstone bridge, having arms 8 and 9 which produce a voltage comparable to the voltage created in arms 6 and 1, the current resulting from the difference in voltages being fed into the input of amplifier l3. A'generator II provides a constant source of current of a frequency f1. As resistance l is constructed with a little less resistance than would be necesssary to provide a balance between arms Ei,l',8 and 9, there is a resulting flow of current through the input wire 5 of amplifier [3. Wire 5 is connected to the wire connecting impedancesiil and 9 and to the wire connecting resistances 5 and l as shown in the drawing. This current in wire 5 energizes amplifier [3 to operate a regulator I4 which controls the output of a generator l2. Regulator l4 therefore includes amplifier I3. Generator [2 is generating a current of a frequency f2 different from the frequency i. Obviously frequency f2 may be direct current, or can be any other different frequency than f1 sufficiently different to be separated therefrom by filters ll, I8 and E9.

The resistance 7, the amplifier is, regulator it, and generator 12 are so proportioned that the current in 5 due to the initial unbalance of bridge 6, l, 8 and 9 will send enough additional current of frequency f2 through resistance l to bring the temperature of the same to a constant tempera ture differential above that of the fluid in pipe Hi. It is understood that as resistance 6 is in a portion of the same fluid at the same temperature in blind conduit lllAas is in conduit is, that changes in the temperature of the fluid will vary the resistance of 6 and! so that current from generator l2 will always maintain the temperature of resistance 1 at a constant temperature differential above that of the fluid. When resistance i is at the selected constant temperature differential above the fluid in conduit ES its resistance has increased thereby approaching that necessary to balance bridge elements 6, l, 8 and 5, which reduced the current in the amplifier input wire 5. Amplifier l3, through regulator M, then cuts down on the amount of current of frequency 2 going to resistance wire 1, which in turn reduces the heating effect in wire i, and thereby the entire system shown in Figure 1 becomes balanced. The balance is reached because a slight reduction in the current through resistance causes a.slight increase in the current in wire 5 and vice versa.

As the. liquid in lilflows it has a greater cool ing effect on resistance 7 so that more current of frequency f2 from generator 52 is required to maintain 1 at a constant temperature differential above that of the fluid. Therefore, the magnitude of the subsidiary current of frequency f2 that must be supplied to resistance 1 when the fluid flows is the function of the velocity or rate of flow of the fluid. Therefore the system shown in Figure 1 is normally balanced with the term perature of resistance 1 above that of resistance 6.

The amount of heat supplied 1, being a function of the velocity or rate of flow, is measured by meter It in the subsidiary circuit. Two different frequencies are used in order that the circuit can be arranged to preventthe heating of 8 by the power supply for the subsidiary heating of "i, and to prevent the introduction of this supply lilPfllallel with 1 from altering the balance of the bridge, as indicated by the meter 16 or amplifier I3. To accomplish this, filters are used. Resonant fllter I8 keeps power of frequency f2 from passing through any arm of the bridge circuit except element 1. Filter I1, identical with filter I8 is inserted to keep the impedance of the arm containing 6 equal to that of the arm containing 1. Tuned filter I9 serves to keep power of frequency f1 from passing through the power source I2 which is in the branch in parallel with 1. I4, responsive to amplifier I3, is the regulator or control unit which maintains the temperature 1 above that of the flowing fluid by a constant amount by regulating the power of frequency f2.

In Figure 2 both temperature sensitive elements 2| and 22 are located in the flowing fluid conduit III. In this circuit filter 26 takes the place of filters I1 and I8 of Figure 1. Filter 26 prevents the power of frequency is from reaching the amplifier I3 which detects the unbalance of the bridge circuit as in Figure 1. Resistances 2| and 22 are both preferably of platinum or like material but the resistance of 2| is chosen such that it is of the order of 10 times that of the resistance of 22. The impedance of 23 to 24 is also of the order of 10 to l in order to provide a bridge balance. Under these conditions only a small amount of heat will be developed in 2| and the temperature of 2| will rise only slightly above the temperature of thenon-fiowing fluid. 2| can thus be used in the flowing fluid, since the cooling produced by the flow will be small as compared to the cooling effect on 22. To prevent too much heating 2| is made of wire of the same or greater cross section as 22 and of greater length, thus increasing the mass of metal to be heated and increasing the surface area for cooling. The velocity of rate of flow is determined by meter I6. I3 indicates the amplifier of the bridge unbalance and I4 is the regulator for maintaining the temperature in 22 above that of the fluid by a constant amount.

Regulator I4 keeps the temperature of resistance 22 above that of the fluid in pipe ID by a constant amount because while both the primary current of frequency f1 and the auxiliary current of frequency f2 are supplied at the same points, namely between the impedances 23 and 24 and between resistances 2| and 22, as resistance 22 is much smaller than the resistance 2! and the impedance 24 is much smaller than impedance 23, most of the current passes through the branch consisting of impedance 24 and re sistance 22 while only 1% of the current passes through the branch consisting of impedance 23 and resistance 2|. The heating effect in a resistance is equal to 1 R where R is the resistance in ohms and I is the square of the current in amps. The size of the currents and resistances involved are such that the square of the current is a larger factor than the size of the resistance, so the bridge 2|, 22, 23 and 24 is normally balanced with the temperature of resistance 22 above that of resistance 2|. As resistance 2| is exposed to fluid in pipe III of the same temperature as encounters resistance 22, resistance 22 is maintained at a constant temperature differential above the temperature of the fluid in conduit ID. The system shown in Figure 2 is balanced with bridge 2|, 22, 23 and 24 unbalanced to the extent necessary to supply enough current of frequency f1 through filter 26 to amplifier I3 to supply sufficent current of frequency f2 through regulator It to maintain resistance 22 at a constant temperature differential above that of fluid Ii]. Because the current 22 is greater than in 2| the heating effect of 2| is not appreciable. Generators II and I2 act as filters preventing the passage of current from the other generator, however, to make this evident in the drawing this filtering function is repeated by filters 25 and 25A, which obviously may be separate units, if desired.

In Figure 3 both temperature sensitive elements 21 and 28 are used in the flowing fluid in conduit ID and both having approximately the same resistance, which also implies that the impedance of 29 and 3| are the same. In this particular circuit element 21 can rise in temperature only a very small amount above that of the fluid, thus permitting the use of 21 in the flowing fluid. The power from source II of frequency fl is used to energize the bridge at a very low level, so that 21 will not be heated appreciably, even though 21 has a resistance of the order of 28. An impedance 32 in series with impedance 29, and shorting switch 30, is used to provide a means of unbalance to the bridge circuit, following the initial balancing With 21 and 28 in non-flowing fluid. Such unbalance is maintained at a predetermined level in the circuit after the switch 30 is opened, by heating 28 with power of frequency f2 from source I2. The value of 32 is chosen to give the desired constant temperature differential AT between 28 and the fluid. When 28 is in the flowing fluid, the power required to maintain the temperature AT is a function of the fluid velocity. It can readily be seen that an auxiliary resistance similar to 32 could be used with the circuit shown in Figure l with suitable bridge resistances.

The system shown in Figure 3 with switch 33 open adds impedance 32 in series to 29 which reduces the current and heating effect in branch 32, 29 and 21 and increases the current and heating effect in branch 28 and 3|. At the same time because the impedance of 29 and 32 unbalances the bridge 29, 32, 21, 28 and 3| a current of frequency ii is supplied to the input of amplifier I3 which results in a heating current of frequency f2 being supplied to the bridge. The major portion or this current it passes through resistance 28, because branch 28 and 3! has less impedance than branch 21, 29 and 32, so resistance 28 is heated a constant amount above the fluid in conduit II), which increases the resistance of 28 and thereby balances the system shown in Figure 3 with. an unbalanced bridge 21, 28, 29, 3| and 32 and a current f2 in resistance 23 from generator I2. The function of filters 25 and 25A is the same in Figure 3 as in Figure 2.

In the above described flow meter circuits the power sources can be maintained at the surface with suitable connections between the sources and the flow meter by use of a multiple wire cable. By suitable arrangement the power sources may be included in the flow meter structure and the entire unit be lowered into the drill hole.

In the operation of the circuit of Figure 1 assumin that impedances 8, 9 are of equal ohmic value, and that there is no flow through. conduit I l, amplifier I3 and regulator Is cause generator I2 to supply sufiicient heating current to the resistance 1 as to balance the bridge. It will be understood that this heating current increases the ohmic value of resistance 1 until it is equal to that of resistance 6. Furthermore, the heating current produces a reading upon meter I6 representative of zero flow, and increases the temperature of resistance 1 above that of resistance 6 and the fluid in conduits I0, II'IA. The

. l 7. temperaturetassumedby resistance when. they initial heating current passes therethrough may beregarded as a reference. temperature. corresponding to the. ambient temperature of the fluid in conduits l0 and llA, for this reference temperature causes the ohmic value of resistance l to. besuch as to balance the bridge circuit when the fluid is at the ambient temperature under consideration.

When there is a flow of fluid through conduit [0, the flow causes an additional abstraction of heat fromresistance 1 corresponding to the flow rate but not from resistance 6. The temperature of resistance 1, therefore, tends to drop below the reference temperature with resultant decrease in. the resistance thereof and unbalancing of the bridge. Accordingly, amplifier I3 and regulator I4 cause generator 12 tov supply additional heating current to resistance 1 until it again assumes the reference temperature at which the ohmic values of ,resistancesrfi and l are equal and the bridge is balanced. This increased heating current increases the reading of meter 56 by an amount corresponding to the rate of flow of fluidthrough conduit lii.-

If the ambient temperature of the fluid changes, the ohmic values of resistances 6 and i both change by a corresponding amount so that such changes do not afiect the balancing of the bridge circuit. Thus, a heating current is supplied to resistance 1 at any such changed ambient temperature under zero flow conditions such as to increase its resistance sufficiently to obtain bridge balance and maintain resistance "l at a reference temperature above that of the fluid in conduit it. The characteristics of the res'mtances are so chosen that the heating current under zero flow conditions is the same at any ambient temperature within the range of the apparatus. Similarly, at any such ambient temperature, flow of fluid through conduit It produces a corresponding incremental abstraction of heat from resistance l' causing its resistance to tend to drop with resultant unbalancing of thebridge. This causes an increase in heatingcurrent through meter l6 and resistance 2' sufficient to rebalance the bridge, the increase in current flowing through the meter corresponding to the flow rate through conduit it. Thus, the current of frequency f1 automatically produces a signal in amplifier or sensing means 13 when the bridge is unbalanced, the sensing means controlling the application of the current of frequency f2 to effect the localized heating of resistance 1 until the bridge is balanced.

If impedances 8, 9 areof unequal ohmic value, the foregoing remarks still apply with full force and effect except that the bridge is balanced when the ohmic values of resistances 6, i have a predetermined relationship to each other, rather than When they are equal.

Inthe operation of the circuit of Figure 2, assuming that there is no flow of fluid through conduit [0, when the bridge is balanced, a heating current flows through meter l6 representative of the no flow conditions, this current being zero in some cases. Changes in ambient temperature of the fluidaffect the temperature and ohmic value of resistances 2|, 22 in proportion to their respective ohmic values, and do not cause any unbalance of the bridge. That is, the circuits are independent of variations in ambient temperature of thefluid in conduit ID. Under these conditions, resistance 22 is at a higher temperature than resistance 2| since the current flow therethrough. is. considerably greater, and the heating efiectvariesas the square of .the current. However, whenfluid flows through. conduit l6, itabstracts. a greater quantityof heat, proportionately, from. resistance 22. than from resistance 2| since, as stated,. resistance 22 is maintained at a. higher temperature than resistance 2| and the former. resistance is specially constructedso as to. lose heat readily to the lowing fluid. Accordingly, an unbalance voltage is pro-v duced which. causes actuationof generator [2 to increase the current of. frequency f2 flowing through meter l5 and the bridge. This additional current has a greater eifect upon resistance 22 than upon resistance 2! so that, when resistance 22 reaches its original temperature as a result of the additional heating current. flowing therethrough, at. which its ohmic value is such as to balance the bridge, the additional current flow through meter I6 produces a reading indicative of the flow ratethrough. conduit Iii. This additional heating current does not effect the regulator l4 due to the. action of filter 26. That is, the current. of frequency fl is utilized primarily to enable sensing of the balanced or unbalanced condition of thebridge while the current of frequency f2 effects. a localized heating of resistance 22.

The operation of the circuit of Figure 3 is generally similar to that of Figure 2. Inthiscircuit, the bridge is balanced With switch 3 5 closed. Changes in ambient temperature do not affect the balance of the bridge since they have a similar effect upon the equal resistances 2i and 23. When switch 30 is opened, the bridge is unbalanced by a predetermined amount, and more current flows'through resistance 28 than through resistance 21, thus increasing the temperature of resistance 28. When this predetermined amount of unbalance occurs, the amplifier i 3 and regulator M- are so adjusted that no current, or a preselected current, flows through meter it, giving a zero flow rate indication. When fluid flows through conduit l0, more heat is abstracted from resistance 28 thanfrom resistance 27, since the former resistance is. at a higher temperature; This tends to decrease the ohmic value of resistance 28 and thereby decrease the unbalance voltage fed to amplifier l3 and regulator Id. As a result, the regulator causes additional current of frequency f2 to flow through meter 36 and the bridge until the original unbalanced condition is restored. The additional current passing through meter lfiis indicative of the flow rate of fluid through conduit 10.

The power measuring meter may read only electrical units and the-velocity or rate of flow be determined from a calibrated curve for the par ticular meter. The meter may be self-recording or non-self-recording, and be maintained at the surface orincludedin. the now meter unit which is lowered into the drill hole.

The flow meter disclosed herein preferably may be included in an inflatable rubber or elastic sealing means as shownin. Figure 4.. The now meter unit 34 issituated in .the center of the collapsible toroidal elastic .membrane 35. Passing through the packer 36 and the flow meter unit 36 is'the conduit Hi which is in communication with. the fluid in the Well above and below the packer 36. A blind conduit IGA is provided as abranch of conduit I0 for use When the system of Figure 1 is employed, in which case resistance 6 is located in branch conduit 1A.. Resistance 7 of Figure '1, 2| and 22 of Figure 2, and 21 and 28 of Figure 3 are all located in conduit 10. In Figure is shown the sealing means 36 in expanded position sealing the flow meter 34 in a drill hole 31 and preventing fluid from flowing between 34 and the walls 38 of the drill hole 31, 3'! represents a support means attached to the flow meter 34 and to the multiple wire cable 4|. 42 represents a tubing through which the sealing means 34 is inflated from the surface. 7

In operation the flow meter is lowered to a point in the drill hole where it is desired to test for fluid flow either from the strata, or into the strata. The sealing means inflated shuts off that part of the drill hole from the surface. If the fluid is coming from a lower strata none can get by the fiow meter and sealing means without, passing throughthe ,flow meter, thus providing an accurate check on the strata below the same. If the flow meter is used in a water input well, such as in secondary recovery programs, the same procedure is used but the rate of flow of fluid into the lower strata is determined. It is intended that two such sealing means may be used (not shown) when it is desired to check the flow of fluid at a particular point in a drill hole, by packing off above and below the point in question. In such event, the flow measuring means would be in the upper inflatable seal. It is obvious that other types or" flow meters could be used with the disclosed sealing means, but the types shown in Figures 1 to 3 are preferred.

It is understood that the circuits described in connection with this invention are to be taken as preferred examples of the same, and that various obvious changes may be made without departing from the invention, the scope of which is defined in the following claims.

Having described our invention, we claim:

1, A flow meter for measuring the rate of flow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a wire connected in shunt with the second and third of said arms, a heating element in said second arm, means to heat said heating element comprising a second source of current, a conductor connecting said second source of current in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, filters isolating said second source of current, said heating element and said conductor from the remainder of the flow meter to prevent said second source of current from heating said first, third and fourth arms, filter means in said. conductor preventing said first source of current from influencing said meter, means exposing said first arm. to a stationary portion of said fluid, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow. 2. A. flow meter for measuring the rate of flow of, a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a wire connected in shunt with the second and third of said arms, a heating element in said second arm, means to heat said heating element comprising second source of current, a conductor connecting said second source of current in series with's'aid heating element, a regulatorin said conductor controlled by said amplifier, a meter in said conductor, filter means in said conductor preventing said first source of current from influencing said meter, means exposing said first arm to a stationary portion of said fluid, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow.

3. A flow meter for measuring the rate of flow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source or current connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a wire connected in shunt with the second and third of said arms, a heating element in said second arm, means to heat said heating element comprising a second source of current, a conductor connecting said second source of'current in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, filters isolating said second source of current, said heating element and said conductor from the remainder of the flow meter to prevent said second source of current from heating said first, third and fourth arms, means exposing said first arm to a stationary portion of said fluid, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow.

4. A flow meter for measuring the rate of flow of a flowing fluid comprising a Wheatstone brige having four arms connected in series, a first source of current connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a wire connected in shunt with the second and third or said arms, a heating element in said second arm, means to heat said heating element comprising a second source of current, a conductor connecting said second source of current in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, filters isolating said second source of current, said heating element and said conductor from the remainder of the flow meter to prevent said second source of current from heating said first, third and fourth arms, filter means in said conductor preventing said first source of current from influencing said meter, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow.

5. A flow meter for measuring the rate of fiow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current-connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a wire connected in shunt with the second and third of said arms, a heating element in said second arm, means to heat said heating element comprising a second source of current, a conductor connecting said second source of current in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, means exposing said first arm to a stationary portion of said fluid, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow.

6. A fiow meter for measuring the rate of flow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current connected in shunt with 11 theflrstand second of said arms, an amplifier havinganv input circuit coupled to a wire connected in shunt with the second and third of said arms, a heating element in said second arm, means toheat said heatingelement -comprising a second source of current, a conductor connecting saidsecond source of current'in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, filter means in said conductor preventing said first source of Current from influencingsaid meter, and means exposing said second arm to said flowing fluid, whereby said meter indicates a parameter of said rate of flow.

7. A flow meter for measuring the rate of flow of a flowing fluid {comprising a 'Wheatstone bridge having ionrarms connected in series, a first sourceof current connected in shunt with the first and second of said arms, an amplifier havingan input-circuit coupled toa wire connected in shuntwith the second and third of said arms, a heating element in said second arm, means to heat sa id heating element comprising a second source of current, a conductor connecting said second source of current in series with said heating element, a regulator in said: conductor controlled by said amplifier, a meter in said conductor, filters isolating said second source of current sa'idheating element and said conductor from the remainder of the flow meter to prevent said second source of current from heating said first, third and fourth arms, and means exposing said second armfto'said flowing fluid, whereby said meter indicates a parameter of said rate of flow 8. A flow meter ior measuring the rate of flow of a flowing fluid comprising a Wheatstone bridge having four arms conn'ected in series, a first source of currentlconnected in shunt 'with the first and-second said arms, an amplifier having an input circuit coupled to a wire connected inshu nt with the second and third of said arms, a heating element in said second arm, meansito heat said iheating element comprising a second source of 'cnrrent, :a conductor connecting said second source of current in series with said heating element, a regu-lator in said conductor controlle'd by said amplifier, a meter in said filters isolating said second source goi currentysaid heating element and said conductor from thefremainder of the flow meter to prevent said second source of current from heating said first, tliird'and fourth arms, filter means in said =-conductor preventing said first source or: current fromfinfluencing said meter, means exposing one of saidarms, which arm is one immediately'iadjacent and connected directly to saigl second arm, to a stationary portion of said fluid,'and means exposing said second arm'to said flowingfluid, whereby said meter indicates a parameter of said rate of flow.

9. A flow meterior'measuring'the rate'of flow of a flowing "fluid ;co mpri'sing' a Wheatstone bridge having four arm connected in series, a first source of current connected in shunt with the firstand secfond'of said arms, anamplifler having an input circuit "coupled to 'a'wire connected in shunt with the second and third of said arms, a heatingelement in said second'arm, means to heat said heating element comprising a second sourceofcurrent; a conductor connecting said second source of current in series with said heating element, a'regulator in said conductor controlledjbysaid amplifierja meter in said conductonfllter means in said conductor preventing said first source of current from Vine. fluencing said mterfmeans exposing "one for said arms, which arm is one immediately J adjacent and connected directly to'said'secondarm, to'a stationary portion of said fluid and means exposing saidsecond arm to'sa'id flowing fluid, whereby said 'meterindi'cates a "p'arameter'of'said rate of flo'w.

10. A flow'meter'fdrmea'suring the rate'of flo'w of a flowing fluid comprising a Wheatstone bridge having four 'arms 'conne'cted'in seriesya first source o'f'c'urrent connected'in shunt with the first and second of said arms, an amplifier havingan inputc'i'rcuit coupled to awireconnected'in shunt with the's'econdand third or said arms, a heating element in said second "arm, means to heat Saidhea'tiing element comprising a second source of currenh aconductor connecting saidj'seco'nd source of current in series with said heatingelementja regulator in said eonductor controlled by said ampliflerfa meterin said conductor, "filters isolating said second source of current, said heating element and's'aid conductor'fromthe remainder of the flow meter to preventsaid second source'of currentfrom heating said first, third'and fourth arms, "means exposing one of said arms, which "arm "is'one immediately adjacent and connected directly to said second arm, to a stationary portion of said fluid, andmeans exposing said second arm to said flowing fluid, whereby said meter'indic'ates a parameter or saidr'a'te (if flow.

11. .A flow meter for measuring 'therate of flow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current connected in shluntwith the first and second of said arms, anamplifier having an input'c'ircuit coupled" to a 'wire'connected in shunt with the second and third 'of said arms, a heating element "in said secondarm, means to heat said' heating element comprising a second source of currentfa conductor'connecting saidlsecond source of current inser'ies with said heating element, a re'gula'tor in said conductor controlled by 'said"arnplifler, a meter in saidconductor,means exposing one of said :arms, which arm is one immediately adjacent and connected directly to said second arm, toa' stationary portion of said fiu'id, a'nd means "exposing said second arm to 'saidflo'w'ing fluid," whereby said meter indicates a parameter of said rate of flow.

12. A flow meter for measuring the rate'of flow of a flowing fluid comprising a Wheats'tone bridge having four arms' connected in series, a flrst'source of current connected in shuntwith the first and second of saidar'ms, an amplifier having an input circuit coupled to a wire'connected 'in shunt with the secondand thirdof said arms, a heating element insa'id second arm, means to heat said heating element comprising a second's'ou'rc'e ofcurr'ent, a conductor connecting said second sourcexorcurrem iin'seri'es with .said'iheating element, a"regulator insaid conductor controlled "by said amplifier, a meter in said conductor, 'fllter' means infsa'id conductor preventing said first source of" current from influencing said "meter, a second heating element in one of saidarms, which arm is one immediately adjacent and connected directly to said second arm, meansl'exposing said second heating elementto saidnowmgnuia, andm'e'ans' exposing said second'arm'to said flowing fluid, the 1mpedancevalues of the arms or the "bridge being so' related that one of said-heating 'elemeritsls normally heated to a higher temperature than the other whereby said meter indicates a parameter of said rate of flow.

13. A flow meter for measuring the rate of flow of a flowing fluid comprising a Wheatstone bridge having four arms connected in series, a first source of current connected in shunt with the first and second of said arms, an amplifier having an input circuit coupled to a Wire connected in shunt with the second and third of said arms, a heating element in said second arm, means to heat said heating element comprising a second source of current, a conductor connecting said second source of current in series with said heating element, a regulator in said conductor controlled by said amplifier, a meter in said conductor, filter means in said conductor preventing said first source of current from infiuencing said meter, a second heating element in one of said arms, which arm is one immediately adjacent and connected directly to said second arm, said second source of current being connected across said second heating element for heating the same, means exposing said second heating element to said flowing fluid, and means exposing said second arm to said flowing fluid, the impedance values of the arms of the WALTER G. MDDLLETON; KENNETH R. MORE].

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,155,630 Savage Oct. 12, 1915 1,291,489 Hadaway Jan. 14, 1919 1,406,682 Rathbone Feb. 14, 1922 1,691,690 Brush et a1 Nov. 13, 1928 2,143,962 Stone Jan. 17, 1939 2,366,351 Patton Jan. 2, 1945 2,437,449 Ames Mar. 9, 1948 OTHER REFERENCES Thermistors, Their Characteristics and Uses by G. L. Pearson, Bell Laboratories Record, Dec. 1940, pages 106-111 (110). 

